Self-suspending proppants for hydraulic fracturing

ABSTRACT

The present invention provides modified proppants, and methods for their manufacture. In embodiments, the modified proppant comprises a proppant particle and a hydrogel coating, wherein the hydrogel coating is applied to a surface of the proppant particle and localizes on the surface to produce the modified proppant. In embodiments, formulations are disclosed comprising the modified particles, and methods are disclosed for using the formulations.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 61/529,600, filed Aug. 31, 2011, U.S. Provisional Application Ser.No. 61/635,612 filed Apr. 19, 2012, and U.S. Provisional ApplicationSer. No. 61/662,681, filed Jun. 21, 2012. The entire contents of theabove-referenced applications are incorporated by reference herein.

FIELD OF APPLICATION

This application relates generally to systems and methods for fracturingtechnologies.

BACKGROUND

In the process of acquiring oil and/or gas from a well, it is oftennecessary to stimulate the flow of hydrocarbons via hydraulicfracturing. The term “fracturing” refers to the method of pumping afluid into a well until the pressure increases to a level that issufficient to fracture the subterranean geological formations containingthe entrapped materials. This process results in cracks and breaks thatdisrupt the underlying layer to allow the hydrocarbon product to becarried to the well bore at a significantly higher rate. Unless thepressure is maintained, however, the newly formed openings close. Inorder to open a path and maintain it, a propping agent or “proppant” isinjected along with the hydraulic fluid to create the support needed topreserve the opening. As the fissure is formed, the proppants aredelivered in a slurry where, upon release of the hydraulic pressure, theproppants form a pack or a prop that serves to hold open the fractures.

To accomplish the placement of the proppants inside the fracture, theseparticles are suspended in a fluid that is then pumped to itssubterranean destination. To prevent the particles from settling, a highviscosity fluid is often required to suspend them. The viscosity of thefluid is typically managed by addition of synthetic or naturally-basedpolymers. There are three common types of polymer-enhanced fluid systemsin general use for suspending and transporting proppants duringhydraulic fracturing operations: slickwater, linear gel, and crosslinkedgel.

In slickwater systems, an anionic or cationic polyacrylamide istypically added as a friction reducer additive, allowing maximum fluidflow with a minimum of pumping energy. Since the pumping energyrequirements of hydraulic fracturing are high, on the order of10,000-100,000 horsepower, a friction reducer is added to slickwaterfluids to enable high pumping rates while avoiding the need for evenhigher pumping energy. While these polymers are effective as frictionreducers, they are not highly effective as viscosifiers and suspendingagents. Slickwater polymer solutions typically contain 0.5-2.0 gallonsof friction reducer polymer per 1000 gallons of slickwater fluid, andthe solutions have low viscosity, typically on the order of 3-15 cps. Atthis low viscosity, suspended proppant particles can readily settle outof suspension as soon as turbulent flow is stopped. For this reason,slickwater fluids are used in the fracturing stages that have either noproppant, proppant with small particle size, or low proppant loadings.

The second type of polymer enhanced fluid system is known as a lineargel system. Linear gel systems typically contain carbohydrate polymerssuch as guar, hydroxyethylcellulose, hydroxyethyl guar, hydroxypropylguar, and hydroxypropylcellulose. These linear gel polymers are commonlyadded at a use rate of 10-50 pounds of polymer per 1000 gallons oflinear gel fluid. These concentrations of linear gel polymer result in afluid with improved proppant suspending characteristics vs. theslickwater fluid. The linear gel fluids are used to transport proppants,at loading levels of about 0.1 to 1 pound of proppant per gallon offluid. Above this proppant loading level, a more viscous solution istypically required to make a stable suspension.

Crosslinked gel is the most viscous type of polymer-enhanced fluid usedfor transporting of proppant. In crosslinked gel systems, the linear gelfluid as described above is crosslinked with added reagents such asborate, zirconate, and titanate in the presence of alkali. Uponcrosslinking of the linear gel fluid into a crosslinked gel fluid, theviscosity is much higher and the proppants can be effectively suspended.The linear gel and crosslinked gel fluids have certain advantages butthey require a high dose rate of expensive polymer.

Modifications of proppant particles could be used advantageously toimprove their performance in hydraulic fracturing systems. First, if theproppant particles were more buoyant, a less viscous suspension fluidcould be used, which would still convey the particles to the target areabut which would be easier to pump into the formation. Second, it isdesirable that the proppants remain where they are placed throughout thelifetime of the well after they have been injected into a fracture line.If changes within the reservoir during well production force theproppants out of position, production equipment can be damaged, and theconductivity of the reservoir formation can be decreased as thereservoir pores are plugged by the displaced proppants. Third, theproppants in the system should be resistant to closure stress once theyare placed in the fracture. Closure stresses can range from 1700 psi incertain shale gas wells, up to and exceeding 15,000 psi for deep, hightemperature wells. Care must be taken that the proppants do not failunder this stress, lest they be crushed into fine particles that canmigrate to undesirable locations within the well, thereby affectingproduction. Desirably, a proppant should resist diagenesis duringfracture treatment. The high pressures and temperatures combine with thechemicals used in frac fluids can adversely affect the proppantparticles, resulting in their diagenesis, which can eventually producefine particulate matter that can scale out and decrease the productivityof the well over time.

Current proppant systems and polymer-enhanced fracturing fluids endeavorto address these concerns, so that the proppants can be carried by thefracturing fluids, can remain in place once they arrive at their targetdestination, and can resist the closure stresses in the formation. Oneapproach to preparing suitable proppants includes coating the proppantmaterials with resins. A resin-coated proppant can be either fully curedor partially cured. The fully cured resin can provide crush resistanceto the proppant substrate by helping to distribute stresses among thegrain particles. A fully cured resin can furthermore help reduce finemigration by encapsulating the proppant particle. If initially partiallycured, the resin may become fully cured once it is placed inside thefracture. This approach can yield the same benefits as the use of aresin that is fully-cured initially.

Another approach to preparing suitable proppants involves mixingadditives with the proppant itself, such as fibers, elastomericparticles, and the like. The additives, though, can affect therheological properties of the transport slurry, making it more difficultto deliver the proppants to the desired locations within the fracture.In addition, the use of additives can interfere with uniform placementof the proppant mixture into the fracture site.

In addition, there are health, safety and environmental concernsassociated with the processing of proppants. For example, fineparticulates (“fines”), such as crystalline silica dust, are commonlyfound in naturally occurring sand deposits. These fines can be releasedas a respirable dust during the handling and processing of proppantsand. With chronic exposure, this dust can be harmful to workers,resulting in various inhalation-associated conditions such as silicosis,chronic obstructive pulmonary disease, lung cancers in the like. Inaddition to these health effects, the fines can cause “nuisance dust”problems such as fouling of equipment and contamination of theenvironment.

While there are known methods in the art for addressing the limitationsof proppant systems, certain problems remain. There is thus a need inthe art for improved proppant systems that allow precise placement,preserve fracture conductivity after placement, protect well productionefficiency and equipment life, and promote worker health and safety. Itis further desirable that such improved systems be cost-effective.

SUMMARY

Disclosed herein, in embodiments, are modified proppants, comprising aproppant particle and a hydrogel coating, wherein the hydrogel coatingis applied to a surface of the proppant particle and localizes on thesurface to produce the modified proppant. The hydrogel coating cancomprise a water-swellable polymer. In embodiments, the hydrogel coatingis applied to the surface as a liquid, which can comprise a solvent or acarrier fluid; the liquid hydrogel coating can become a dried hydrogelcoating by removal of the solvent or the carrier fluid. In embodiments,the hydrogel coating comprises a water-swellable polymer that respondsto elevated temperatures or brine conditions by collapsing in volume orthickness. In embodiments, the hydrogel coating comprises a hydrophobiccomonomer selected from the group consisting of alkyl acrylate esters,N-alkyl acrylamides, N-isopropylacrylamide, propylene oxide, styrene,and vinylcaprolactam. In embodiments, the dried hydrogel coating iscapable of expanding in volume in contact with an aqueous fluid to forma swollen hydrogel coating having a thickness of at least about 10%greater than the dried hydrogel coating. In embodiments, the hydrogelcoating comprises a polymer selected from the group consisting ofpolyacrylamide, poly(acrylic acid), copolymers of acrylamide withacrylic acid salts, carboxymethyl cellulose, hydroxyethyl cellulose,hydroxypropyl cellulose, guar gum, carboxymethyl guar, carboxymethylhydroxypropyl guar gum, hydrophobically associating swellable emulsionpolymers, and latex polymers. In embodiments, the hydrogel coatingfurther comprises chemical additives selected from the group consistingof scale inhibitors, biocides, breakers, wax control agents, asphaltenecontrol agents, and tracers.

In embodiments, the modified proppant further comprises acationic/anionic polymer pair comprising a cationic polymer and a highmolecular weight anionic polymer; the cationic polymer can be selectedfrom the group consisting of poly-DADMAC, LPEI, BPEI, chitosan, andcationic polyacrylamide. In embodiments, the modified proppant furthercomprises a crosslinking agent; the crosslinking agent can comprise acovalent crosslinker, and the covalent crosslinker can comprise afunctional group selected from the group consisting of an epoxide, ananhydride, an aldehyde, a diisocyanate, and a carbodiamide. Inembodiments, the covalent crosslinker can be selected from the groupconsisting of polyethylene glycol, diglycidyl ether, epichlorohydrin,maleic anhydride, formaldehyde, glyoxal, glutaraldehyde, toluenediisocyanate, and methylene diphenyl diisocyanate,1-ethyl-3-(3-dimethylaminopropyl)carbodiamide. In embodiments, themodified proppant can further comprise a delayed hydration additive; thedelayed hydration additive can be selected from the group consisting ofa low hydrophilic-lipophilic balance surfactant, an exclusion agentcapable of excluding a finishing surfactant, a light ionic crosslinkingagent, a light covalent crosslinking agent and a monovalent salt chargeshielder. In embodiments, the modified proppant further comprises analcohol selected from the group consisting of ethylene glycol, propyleneglycol, glycerol, propanol, and ethanol. In embodiments, the modifiedproppant further comprises an anticaking agent.

Also disclosed herein are hydraulic fracturing formulations comprisingthe modified proppant described above. In embodiments, the formulationscan further comprise uncoated sand and/or fibers. Methods are disclosedherein, in embodiments, for fracturing a well, comprising preparing thehydraulic fracturing formulation described above, and introducing thehydraulic fracturing formulation into the well in an effective volumeand at an effective pressure for hydraulic fracturing, therebyfracturing the well.

Also disclosed herein, in embodiments, are methods of forming a modifiedproppant, comprising providing a proppant particle; and applying ahydrogel coating to a surface of the proppant particle so that thehydrogel coating localizes on the surface. In embodiments, the hydrogelcoating is applied to the surface as a liquid. The methods can furthercomprise comprising the step of drying the hydrogel coating on thesurface by a drying process, which can comprise heating the hydrogelcoating. In embodiments, the hydrogel coating comprises a solvent or acarrier fluid, and the hydrogel coating dries on the surface by removalof the solvent or the carrier fluid to form a dried hydrogel coating. Inembodiments, the method can comprise the further step of exposing thedried hydrogel coating to an aqueous fluid to form a swollen hydrogelcoating, wherein the swollen hydrogel coating expands in volume to havea thickness of at least about 10% greater than the thickness of thedried hydrogel coating.

In addition, methods are disclosed herein for manufacturing a modifiedproppant, comprising providing a proppant substrate particle and a fluidpolymeric coating composition, applying the fluid polymeric coatingcomposition on the proppant substrate particle, mixing the proppantsubstrate particle and the fluid polymer coating composition to form amodified proppant, and drying the modified proppant, wherein the fluidpolymeric coating composition comprises a hydrogel polymer, and whereinthe hydrogel polymer localizes on the surface of the proppant substrateparticle to produce the modified proppant. In embodiments, themanufacturing takes place at or near a point of use for the modifiedproppant. In embodiments, the proppant substrate particle comprisessand. In embodiments, the sand is obtained at or near the point of usefor the modified proppant. These methods can further comprise adding analcohol selected from the group consisting of ethylene glycol, propyleneglycol, glycerol, propanol, and ethanol during or before the step ofmixing the proppant substrate particles and the fluid polymer coatingcomposition. These methods can further comprise adding an inversionpromoter during or following the step of mixing the proppant substrateparticles and the fluid polymer coating composition. These methods canfurther comprise adding an anticaking agent to the modified proppant.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a flow diagram of a manufacturing process for self-suspendingproppants.

FIG. 2 is a graph of bed height vs. shear time for three sets ofself-suspending proppant samples.

FIG. 3 is a graph of bed height vs. mixing time for two sets ofself-suspending proppant samples.

FIG. 4 is a graph of bed height vs. mixing time for two sets ofself-suspending proppant samples.

FIG. 5 is a graph of bed height vs. mixing time for a series of treatedself-suspending proppant samples.

FIG. 6 is a graph of bed height for varying amounts of calcium silicateadded to self-suspending proppant samples.

FIG. 7 is a graph of bed height vs. drying time for a series ofpreheated and non-preheated proppant samples.

FIG. 8 shows a graph of bed height vs. drying time at varioustemperatures.

FIG. 9 shows a graph of temperature vs. mixing time for a series oftreated self-suspended proppant samples.

FIG. 10 shows a graph of bed height and loss of ignition (LOI) vs.drying time.

DETAILED DESCRIPTION

1. Modified Proppant Particles

Disclosed herein are systems and methods for forming and using proppantparticles having a hydrogel surface layer to enhance the hydrodynamicvolume of the proppant particles during fluid transport, creating a morestable proppant suspension that resists sedimentation, separation, andscreenout before the proppant can reach the intended target destinationin the fracture. Further benefits of the hydrogel-coated proppants asdisclosed herein include lower tendency to erode equipment, lowerfriction coefficient in the wet state, good bonding adhesion with eachother after placement in a fracture site, resistance to uncontrolledfines formation, and anti-fouling properties attributable to thehydrophilic surface. In embodiments, the disclosed systems for formingproppant particles can be applied to the types of proppant substratesmost widely used, e.g., sand, resin coated sand, and ceramics. In otherembodiments, the proppant particles can be formed from a variety ofsubstrates, including fibrous materials, as would be available to thosehaving ordinary skill in the art. In certain embodiments, the proppantparticles can be fabricated so that they resist crush or deformation, sothat they resist displacement, and so that they can be suspended in lessviscous fluid carriers for transporting into the formation.

The invention encompasses a modified proppant, comprising a proppantparticle and a hydrogel coating, wherein the hydrogel coating localizeson the surface of the proppant particle to produce the modifiedproppant. In embodiments, these self-suspending proppants are formed bymodification of a particulate substrate with a water swellable polymercoating such as a hydrogel. In embodiments, the particulate substratecan be modified with the polymer coating before the particulatesubstrate is introduced into the fracturing fluid. In embodiments, theamount of hydrogel polymer coating can be in the range of about 0.1 toabout 10% based on the weight of the proppant. In embodiments, thehydrogel layer applied onto the surface of the proppant substrate can bea coating thickness of about 0.01% to about 20% of the average diameterof the proppant substrate. Upon hydration and swelling of the hydrogellayer in the fracturing fluid, the hydrogel layer can become expandedwith water, such that the hydrogel layer thickness can become about 10%to about 1000% of the average diameter of the proppant substrate.

Methods for modification of proppant include spraying or saturation of aliquid polymer formulation onto a proppant substrate, followed by dryingto remove water or other carrier fluids. The drying process can beaccelerated by application of heat or vacuum, and by tumbling oragitation of the modified proppant during the drying process. Theheating can be applied by forced hot air, convection, friction,conduction, combustion, exothermic reaction, microwave heating, orinfrared radiation. Agitation during the proppant modification processhas a further advantage of providing a more uniform coating on theproppant material.

FIG. 1 illustrates schematically a manufacturing process 100 forpreparing self-suspending proppant 130 in accordance with the presentdisclosure. In the depicted embodiment, sand 132 (e.g., dry sand havingless than 0.1% moisture) is conveyed via a conveyor belt 122 into amixing vessel 124, and a liquid polymer composition 120 is sprayed viapump and spray nozzle apparatus 134 onto the sand 132 along the conveyorbelt 122. The sand 132 exposed to the liquid polymer 120 reports to alow shear mixing vessel 124, where the ingredients are further blendedto form modified sand 128. After mixing, the modified sand containingthe liquid polymer is sent to a dryer 126 to remove water and/or organiccarrier fluids associated with the liquid polymer 120. After the dryingstep, the dried modified sand 132 is passed through a finalizing step134, which can include a shaker and/or other size classificationequipment such as a sieve to remove over-sized agglomerates. Thefinalizing step 134 can also subject the dried modified sand 132 tomechanical mixers, shear devices, grinders, crushers or the like, tobreak up aggregates to allow the material to pass through theappropriate sized sieve. The finished material 130 is then stored forshipment or use.

In embodiments, the sand that is used to produce self-suspendingproppant is pre-dried to a moisture content of <1%, and preferably <0.1%before being modified with a hydrogel polymer. In embodiments, the sandtemperature at the time of mixing with the liquid polymer is in therange of about 10 to about 200 degrees C., and preferably in the rangeof about 15 to about 80 degrees C.

In embodiments, the sand is contacted with the liquid polymercomposition by means of spraying or injecting. The amount of liquidpolymer composition added is in the range of about 1 to about 20%, andpreferably about 2 to about 10% by weight of the sand. The sand andliquid polymer are blended for a period of 0.1 to 10 minutes. In apreferred embodiment, the mixing equipment is a relatively low sheartype of mixer, such as a tumbler, vertical cone screw blender, v-coneblender, double cone blender, pug mill, paddle mixer, or ribbon blender.In embodiments, the mixing equipment can be equipped with forced air,forced hot air, vacuum, external heating, or other means to causeevaporation of the carrier fluids.

In embodiments, the modified sand containing the liquid polymer is driedto remove water and/or organic carrier fluids associated with the liquidpolymer. The dryer equipment can be a conveyor oven, microwave, orrotary kiln type. In an embodiment the drying step is carried out insuch a way that the dried, modified sand contains less than 1% by weightof residual liquids, including water and any organic carrier fluidsassociated with the liquid polymer composition.

In embodiments, the same equipment can be used to blend the sand withthe liquid polymer and to dry the blended product in a single processingstage, or in a continuous production line. In an embodiment, the processof converting a substrate such as sand into a self-suspending proppantcan be conducted at or near the point of use, for example at an oil orgas well site in preparation for hydraulic fracturing. This method hasthe advantage of converting a commodity material with high materialhandling costs, such as sand, into a specialized material that has addedfeatures. The sand can be acquired from local sources or shippeddirectly from a sand mining site or warehouse, for modification at thepoint of use. This avoids having to ship sand first into a blendingplant and then ship a second time from the blending plant to the pointof use. In the case of sand, the shipping costs can be higher than thematerial costs, so avoidance of extra shipping is desirable forcontrolling costs.

In an exemplary manufacturing process, the sand and the modifyingchemicals can be added to a continuous mixer. After mixing is complete,the mixture can either be (a) ready to use or (b) sent to a drying step.The drying step can include a thermal or vacuum drying process, and itcan include the addition of anticaking agents. The finished product canbe stored in containers at the well site. An example of the mixingequipment is a continuous ribbon blender or a pug mill. The drying stepcan be a separate process from mixing, and the drying step can bedesigned to avoid overshearing of the finished product such as aconveyor or tunnel dryer. Other types of drying mechanisms includerotary kilns, microwave driers, paddle driers, and vacuum driers.

Hydrogel polymers that can be used to modify proppants in accordancewith the systems and methods disclosed herein can be introduced, inembodiments, as oil-based emulsions, suspensions, water-based emulsions,latexes, solutions, and dispersions. In embodiments, the hydrogelpolymers can be introduced as a distilled emulsion, such as an oil basedemulsion that has been partially evaporated to remove a portion of thecarrier fluids. This can offer the advantage of reduced dryingrequirements compared with conventional emulsions. In embodiments, thehydrogel polymer can be an alkali-swellable emulsion, wherein thehydrogel properties of the polymer are not fully developed until thepolymer is contacted with alkali. In this embodiment, thealkali-swellable emulsion can be coated onto the proppant substrate toform a modified proppant, and this modified proppant can be suspended ina fracturing fluid in the presence of an alkaline material.

In embodiments, an additive such as an alcohol selected from the groupconsisting of ethylene glycol, propylene glycol, glycerol, propanol, andethanol can be added during or before the step of mixing the proppantsubstrate particles and the liquid polymer coating composition. Inembodiments, inversion promoters useful as additives in the polymercoating formulations for self-suspending proppants can include high HLBsurfactants, such as polyethylene oxide lauryl alcohol surfactant,(ETHAL LA-12/80% from ETHOX), ethylene glycol, propylene glycol, water,sodium carbonate, sodium bicarbonate, ammonium chloride, urea, bariumchloride, and mixtures thereof.

In other embodiments, the proppant substrate can be modified with apolymer formulation, without the need for a drying step. This can beaccomplished by the use of a solvent-free polymer formulation, or acurable formulation. In certain simplified methods, a dry or liquidpolymer formulation can be applied onto the proppant substrate viainline mixing, and the modified material thus prepared can be usedwithout further processing. The moisture content of the proppantsubstrate can be modified by addition or removal of water, or additionof other liquids, to allow the substrate to be effectively coated,handled, and delivered into the fracturing fluid.

The modified proppants can be further modified with a wetting agent suchas a surfactant or other hydrophilic material to allow for effectivedispersion into a fracturing fluid. When the hydrogel-modified proppantsare suspended into a fracturing fluid, they are considered to beself-suspending if they require a lower viscosity fluid to prevent theparticles from settling out of suspension.

The modified proppants can be further modified with an anticaking agentsuch as calcium silicate, magnesium silicate, colloidal silica, calciumcarbonate, or microcrystalline cellulose to improve the flowability andhandling properties of the modified proppant material.

The hydrogel-modified proppants of the invention can advantageously usea localized polymer concentration on the proppant surface, in contrastto the traditional approach of making the entire fluid medium viscous.This localized hydrogel layer can permit a more efficient use ofpolymer, such that a lower total amount of polymer can be used tosuspend proppant, as compared, for example, with conventionalpolymer-enhanced fracturing fluids such as slickwater, linear gel, andcrosslinked gel. Although the hydrogel-modified proppants are consideredto be self-suspending, they can be used in combination with frictionreducers, linear gels, and crosslinked gels.

The hydrogel-modified proppants as disclosed herein can have theadvantage of delivering friction-reducing polymer into the fracturingfluid, so that other friction reducer polymers might not be required ormight be required in lesser amounts when the hydrogel-modified proppantsare used in hydraulic fracturing operations. In embodiments, some of thehydrogel polymer can desorb from the surface of the proppant to deliverfriction reducing benefits or viscosity features to the fracturingfluid.

The hydrogel polymer used for preparation of hydrogel-modified proppantscan, in embodiments, comprise polymers such as a polyacrylamide,copolymers of acrylamide with anionic and cationic comonomers,copolymers of acrylamide with hydrophobic comonomers, poly(acrylicacid), poly(acrylic acid) salts, carboxymethyl cellulose, hydroxyethylcellulose, hydroxypropyl cellulose, guar gum, alginate, carrageenan,locust bean gum, carboxymethyl guar, carboxymethyl hydroxypropyl guargum, hydrophobically associating swellable emulsion (HASE) polymers,latex polymers, starches, and the like. In embodiments, the hydrogelpolymer can be crosslinked to enhance the water absorbing and swellingproperties of the polymer. The crosslinkers can be introduced as anelement of the hydrogel base polymer, or they can be introduced aschemical modifiers for pre-formed polymers.

Localizing the polymer around the proppant surface as described hereincan result in a more effective use of polymer and can prevent proppantfrom settling out of a polymer solution. In embodiments, the polymerlayer hydrates around the proppant effectively preventingproppant/proppant (interparticle) contact. This can prevent the proppantfrom forming a compact settled bed and can result in a proppant that iseasier to resuspend in a fracturing fluid. The resuspension propertiesfor the modified proppants can be important if the fluid flow isinterrupted during hydraulic fracturing operations. In this event, whenthe flow is resumed it is important that the proppant can be resuspendedto avoid the loss of proppant or the unintended blockage of a fluidpath.

The polymer surface modifications as described herein can cause anincrease in the effective hydrodynamic radius of the proppant particlewhen the polymer swells. This can result in increased drag on theproppant as well as effectively changing the overall hydrogel/particledensity. Both can result in a proppant particle with a slower settlingrate and superior transport properties.

In embodiments, polymer pairing or ionic crosslinking can be used toimprove the hydrogel polymer retention on the surface of the proppantparticles. For example, a cationic polymer can be deposited onto theproppant as a first layer to “lock in place” a second layer containing ahydrogel such as a high molecular weight anionic polymer. Inembodiments, the cationic polymer can be polydiallyldimethylammoniumchloride (poly-(DADMAC)), linear polyethylenimine (LPEI), branchedpolyethylenimine (BPEI), chitosan, epichlorohydrin/dimethylaminepolymer, ethylene dichloride dimethylamine polymer, or cationicpolyacrylamide. The cationic polymer layer can be placed on the proppanteither before or after proppant surface modification with the anionichydrogel layer. The ionic interaction can act as a crosslink mechanismto help prevent the anionic polymer from desorbing in high shearenvironments such as going through a pump or during pumping down thewellbore. The cationic polymer can also improve polymer retention bycausing a delay in the hydration and extension of the anionic polymerchains. It is believed that less polymer chain extension during thepumping process will yield higher polymer retention on the proppant(i.e. less desorption).

Covalent crosslinking of the hydrogel polymer layer on proppant surfacecan improve the swelling properties of the polymer and the sheartolerance to prevent premature release of the hydrogel from theproppant. Covalent crosslinkers can include the following functionalgroups: epoxides, anhydrides, aldehydes, diisocyanates, carbodiamides,divinyl, or diallyl groups. Examples of these covalent crosslinkersinclude: PEG diglycidyl ether, epichlorohydrin, maleic anhydride,formaldehyde, glyoxal, glutaraldehyde, toluene diisocyanate, methylenediphenyl diisocyanate, 1-ethyl-3-(3-dimethylaminopropyl)carbodiamide,and methylene bis acrylamide. Covalent crosslinking of the hydrogelpolymer layer on the proppant surface can effectively create a swellable“polymer cage” around the proppant. The covalent bonds prevent thepolymer from completely desorbing into solution. The slightly insolublepolymer layer is able to swell and produce a hydrated polymer layer.

Delayed/controlled hydration of polymer layer may be desirable to delaythe hydration of the polymer surface modification during handling of theproppant and initial pump-down through the wellbore. Environmentalfactors such as humidity and rain could cause premature hydration of thepolymeric coating, which would make it difficult to effectively meterthe proppant dose into the blender during a hydraulic fracturingoperation. It is also believed that a fully hydrated polymer layer canbe more prone to desorption under the high shear conditions associatedwith pumping of a fracturing fluid down the tubular. For these reasons,it may be advantageous to engineer a surface-modified proppant havingslower or delayed hydration properties. In embodiments, delayedhydration can be achieved by addition of a low hydrophilic-lipophilicbalance (HLB) surfactant, exclusion of a high HLB finishing surfactant,ionic crosslinking, covalent crosslinking, charge shielding using amonovalent salt, or by incorporation of a hydrophobic layer such as afatty acid, or a fatty alcohol.

In embodiments, hydrophobic groups can be incorporated into the hydrogelpolymer to allow for hydrophobic interactions. This method can improvethe salt tolerance of the hydrogel layer, such that the hydrogel layerremains swellable even in an aqueous fluid that contains elevated saltconcentrations.

Also disclosed herein is a method of fracturing a well using a hydrogelcoated proppant in combination with non-hydrogel-coated proppant. Forexample, the hydrogel-coated proppant can serve as a suspending agentfor the non-hydrogel-coated proppant.

Also disclosed herein is a method of improving well productivity byimproved proppant placement using a hydrogel-coated proppant. Thehydrogel-coated proppant can be more effectively transported into thefar end of fractures to enable higher productivity of oil and gas from awell. Because the surface-modified proppants disclosed herein can beless inclined to settle out of the fluid and easier to resuspend andtransport through the fracture, it is believed that proppant placementwill be more effective. The ability to transport proppant further intofractures could significantly increase the effectiveness of a fracturingstimulation operation, resulting in a larger of volume of higher densityfractures. These fracture channels can advantageously allowgas/condensate to more easily flow into the wellbore from the reservoir.

Also disclosed herein is an improved method of proppant placement usinga low viscosity fluid. The surface modified proppants as disclosedherein utilize polymers more effectively to suspend/transport proppantparticles. The surface modification renders the proppantself-suspending, thereby reducing or eliminating the need for highlyviscous fluids/gels to transport proppant. Hence, lower viscosity fluidscan be used in combination with the surface-modified proppant totransport proppant into fractures. This would advantageously simplifythe formulation of fracturing gels for use with proppants.

Also disclosed herein is a more efficient method of fracturing a wellusing less proppant. Because highly effective proppant placement can beachieved with the easily-transportable surface-modified proppants asdisclosed herein, it is anticipated that a smaller amount of thesesurface-modified proppants would be required for any given fracturingoperation, as compared to systems using traditional proppants. With anincreasing demand for fracturing grade sand/proppants, and a decreasingsupply of desirably-shaped sand for proppant use, it would beadvantageous to provide systems and methods such as those disclosedherein where less proppant can be used to achieve results comparable toor superior to the outcomes using current techniques.

After the hydrogel coated proppants of the invention have been pumpedinto a well, the hydrogel layer can be degraded by chemical, thermal,mechanical, and biological mechanisms. Specifically, the polymericsurface modification on the proppant can be broken down with the aid ofchemical breakers, for example, ammonium persulfate, magnesium peroxide,or other oxidizers. The polymeric surface modification on the proppantcan also be broken down with the aid of ambient reservoir conditions,such as elevated brine content, elevated temperature, and contact withhydrocarbons. Controlled breaking of the hydrogel layer upon reaching atarget temperature or amount of time in the fluid, can be used as ameans to direct the placement of the proppant in the desired location infractures. The degradation of the hydrogel layer is also beneficial toensuring the adequate conductivity of the propped fracture aftercompleting the hydraulic fracturing operations. In embodiments, thehydrogel layer can demonstrate stimuli-responsive properties, so that itswells with water when exposed to a first set of conditions, such as acertain first temperature or pH, and it loses water, loses volume, losesthickness, or even collapses, when subjected to a certain set ofconditions, such as a second temperature or pH.

For example, in an embodiment, temperature-responsive hydrogels can becoated onto proppant materials. The temperature responsive hydrogellayer can swell when exposed to water at a first set of conditions, suchas a water temperature of 50-100 degrees F., and then it can collapsewhen exposed to a second set of conditions, such as a water temperatureof 110-450 degrees F. Using this stimuli-responsive mechanism, thetemperature responsive hydrogel coated proppant can have self-suspendingproperties as the fracturing fluid carries it underground to thelocation of the fractures at an initial water temperature, for example50-100 degrees F. As the coated proppant encounters the highertemperature region of the underground formation, such as 110-450 degreesF., the hydrogel layer can collapse, allowing deposition andconsolidation of the proppant in the fissures. The temperatureresponsive hydrogel can be a water soluble polymer or copolymercomposition comprising hydrophobic monomers selected from the groupconsisting of alkyl acrylate esters, N-alkyl acrylamides, propyleneoxide, styrene, and vinylcaprolactam. The N-alkyl substitutedacrylamides can be N-isopropylacrylamide, N-butylacrylamide,N-octylacrylamide, and the like. The alkyl acrylate esters can besubstituted by alkyl chains having from 1 to about 30 carbons. In apreferred embodiment, the temperature responsive hydrogel polymercomprises N-isopropylacrylamide and contains up to about 90 percent ofhydrophilic comonomer units. The type and amount of the hydrophobicmonomer substituent in the hydrogel polymer can be selected byexperimental optimization techniques to adjust the water solubility andthe temperature responsive properties of the hydrogel polymer.

Also disclosed herein is a method of delivery of chemical additives intothe proppant pack, by incorporating the chemical additives into, orassociated with, the hydrogel layer of the modified proppant. Thechemical additives that can be advantageously delivered via the hydrogellayer include scale inhibitor, biocide, breaker, wax control, asphaltenecontrol, and tracers. The chemical additives can be in the form of watersoluble materials, water insoluble particles, fibers, metallic powdersor flakes, and the like. The chemical additives can be selected suchthat they slowly dissolve or decompose to release their chemicalactivity. In embodiments, chemical additives can be incorporated into,or associated with, the hydrogel layer by physical entrainment,layer-by-layer deposition, covalent attachment, ionic association,hydrophobic association, or encapsulation. The chemical additives can beadded to the proppant separately from the hydrogel, or they can becombined with the hydrogel coating formulation at the time ofmanufacture of the coated proppant. Breaker chemicals such aspersulfates, peroxides, permanganates, perchlorates, periodates orpercarbonates can be added in this method of delivery. The transport anddelivery of these chemicals with the hydrogel coated proppant has theadvantage of a targeted delivery of the chemicals to a fracture or to aproppant pack. This method offers the advantage of concentrating thechemical additives in the location where their function is needed,thereby reducing the overall amount of chemical additives needed.Certain breakers such as peroxides and persulfates have an acceleratedactivity at higher temperatures. Using this method, the breakerchemicals incorporated in the hydrogel layer will become activated uponplacement in the fractures, by the elevated temperatures of thepetroleum bearing reservoir.

In embodiments, the surface of a proppant particulate substrate can becoated with a selected polymer, either as a single layer or as a seriesof multiple coating layers. The coating (either single layer ormultilayer) can show switchable behavior under certain circumstances. Asused herein, the term “switchable behavior” or “switching behavior”refers to a change in properties with a change in circumstances, forexample, a change from one set of properties during the transport phaseand another set of properties inside the fracture. Switching behaviorcan be seen, for example, when a particle demonstrates hydrophilicproperties in the fracturing fluid and adhesive properties when in placewithin the fractures. Such behavior can triggered by circumstances likethe high closing pressures inside the fracture site so that the outerlayer of the coating rearranges itself to exhibit more advantageousproperties.

In an embodiment, the coated particle can switch from hydrophilic tohydrophobic when subjected to the high pressures inside the fractures.In an exemplary embodiment, during the transport phase, when thehydrophilic covering of the particle is exposed to the water-basedfracturing fluid, it will tend to be fully distended. As a result, thecoating can provide the particle with lubrication in this state,facilitating its movement through the proppant slurry. When the particlehas been conveyed to its destination within the fractures in theformation though, the high pressures there will overcome the stericrepulsions of the external hydrophilic polymer chains, forcing the outerlayer to rearrange itself so that the inner layer is exposed. Inembodiments, the switchable inner layer can be hydrophobic or adhesive,or both. As the inner layer becomes exposed, its properties can manifestthemselves. If the inner layer has adhesive properties, for example, itcan fix the particles to each other to prevent their flowback. Thisinner layer can also be configured to capture fines in case the proppantparticle fails. Moreover, the residual intact hydrophilic groups presentin the outer coating can allow easy flow of oil through the proppantpack.

In embodiments, a coated proppant particle can be produced that bearsthe following layers of coating. First, a pressure-activated fixativepolymer can be used to coat the proppant substrate. This coating layercan be elastomeric, thereby providing strength to the proppant pack byhelping to agglomerate the proppant particles and distribute stress. Inaddition, this coating layer can encapsulate the substrate particles andretain any fines produced in the event of substrate failure. Second, ablock copolymer can be adsorbed or otherwise disposed upon the firstlayer of coating. The copolymer can have a section with high affinityfor the first polymeric layer, allowing strong interaction (hydrophobicinteraction), and can have another section that is hydrophilic, allowingfor easy transport of the proppant in the transport fluid.

In certain embodiments, a stronger interaction between the first andsecond coating layers may be useful. To accomplish this, aswelling-deswelling technique can be implemented. For example, the blockcopolymer can be adsorbed onto the surface of the elastomeric-coatedparticle. Then, the first coating layer can be swelled with smallamounts of an organic solvent that allow the hydrophobic block of thecopolymer to penetrate deeper into the first coating layer and to becomeentangled in the elastomeric coating. By removing the organic solvent,the layered polymeric composite will deswell, resulting in a strongerinteraction of copolymer with the elastomeric particle. A method forswelling-deswelling technique that can be useful is set forth in“Swelling-Based Method for Preparing Stable, Functionalized PolymerColloids,” A. Kim et al., J. Am. Chem. Soc. (2005) 127: 1592-1593, thecontents of which are incorporated by reference herein.

In embodiments, proppant systems using coatings as disclosed herein candecrease the amount of airborne particles associated with proppantmanufacture. For example, respirable dust including fine crystallinesilica dust associated with handling and processing proppant sand can becaptured and held by the proppant coatings during their processing. Inembodiments, coating agents can be added that have a particular affinityfor particulates in the environment that could adversely affect workersafety or create nuisance dust problems. In embodiments, a hydrogelcoating on proppant particles can serve as a binder or capturing agentby mechanically entrapping or adhering to the dust particulates.

While the systems described herein refer to a two-layer coating system,it is understood that there can be multiple (i.e., more than two)coating layers forming the composite proppant particles disclosedherein, with the each of the multiple coating layers possessing some orall of the attributes of the two coating layers described above, or withone or more of the multiple coating layers providing additionalproperties or features.

2. Particulate Substrate Materials

Composite proppant particles in accordance with these systems andmethods can be formed using a wide variety of proppant substrateparticles. Proppant particulate substrates can include for use in thepresent invention include graded sand, resin coated sand, bauxite,ceramic materials, glass materials, walnut hulls, polymeric materials,resinous materials, rubber materials, and the like, and combinationsthereof. The self-suspending proppant (“SSP”) as disclosed herein canalso be made using specialty proppants, such as ceramics, bauxite, andresin coated sand. By combining sand SSP with specialty SSP, a proppantinjection can have favorable strength, permeability, suspension, andtransport properties. In embodiments, the substrates can includenaturally occurring materials, for example nutshells that have beenchipped, ground, pulverized or crushed to a suitable size (e.g., walnut,pecan, coconut, almond, ivory nut, brazil nut, and the like), or forexample seed shells or fruit pits that have been chipped, ground,pulverized or crushed to a suitable size (e.g., plum, olive, peach,cherry, apricot, etc.), or for example chipped, ground, pulverized orcrushed materials from other plants such as corn cobs. In embodiments,the substrates can be derived from wood or processed wood, including butnot limited to woods such as oak, hickory, walnut, mahogany, poplar, andthe like. In embodiments, aggregates can be formed, using an inorganicmaterial joined or bonded to an organic material. Desirably, theproppant particulate substrates will be comprised of particles (whetherindividual substances or aggregates of two or more substances) having asize in the order of mesh size 4 to 100 (US Standard Sieve numbers). Asused herein, the term “particulate” includes all known shapes ofmaterials without limitation, such as spherical materials, elongatematerials, polygonal materials, fibrous materials, irregular materials,and any mixture thereof.

In embodiments, the particulate substrate can be formed as a compositefrom a binder and a filler material. Suitable filler materials caninclude inorganic materials such as solid glass, glass microspheres, flyash, silica, alumina, fumed carbon, carbon black, graphite, mica, boron,zirconia, talc, kaolin, titanium dioxide, calcium silicate, and thelike. In certain embodiments, the proppant particulate substrate can bereinforced to increase their resistance to the high pressure of theformation which could otherwise crush or deform them. Reinforcingmaterials can be selected from those materials that are able to addstructural strength to the proppant particulate substrate, for examplehigh strength particles such as ceramic, metal, glass, sand, and thelike, or any other materials capable of being combined with aparticulate substrate to provide it with additional strength.

In certain embodiments, the proppant particulate substrate can befabricated as an aggregate of two or more different materials providingdifferent properties. For example, a core particulate substrate havinghigh compression strength can be combined with a buoyant material havinga lower density than the high-compression-strength material. Thecombination of these two materials as an aggregate can provide a coreparticle having an appropriate amount of strength, while having arelatively lower density. As a lower density particle, it can besuspended adequately in a less viscous fracturing fluid, allowing thefracturing fluid to be pumped more easily, and allowing more dispersionof the proppants within the formation as they are propelled by the lessviscous fluid into more distal regions. High density materials used asproppant particulate substrates, such as sand, ceramics, bauxite, andthe like, can be combined with lower density materials such as hollowglass particles, other hollow core particles, certain polymericmaterials, and naturally-occurring materials (nut shells, seed shells,fruit pits, woods, or other naturally occurring materials that have beenchipped, ground, pulverized or crushed), yielding a less dense aggregatethat still possesses adequate compression strength.

Aggregates suitable for use as proppant particulate substrates can beformed using techniques to attach the two components to each other. Asone preparation method, a proppant particulate substrate can be mixedwith the buoyant material having a particle size similar to the size ofthe proppant particulate substrates. The two types of particles can thenbe mixed together and bound by an adhesive, such as a wax, aphenol-formaldehyde novolac resin, etc., so that a population of doubletaggregate particles are formed, one subpopulation having a proppantparticulate substrate attached to another similar particle, onesubpopulation having a proppant particulate substrate attached to abuoyant particle, and one subpopulation having a buoyant particleattached to another buoyant particle. The three subpopulations could beseparated by their difference in density: the first subpopulation wouldsink in water, the second subpopulation would remain suspended in theliquid, and the third subpopulation would float.

In other embodiments, a proppant particulate substrate can be engineeredso that it is less dense by covering the surface of the particulatesubstrate with a foamy material. The thickness of the foamy material canbe designed to yield a composite that is effectively neutrally buoyant.To produce such a coated proppant particulate, a particle having adesirable compression strength can be coated with one reactant for afoaming reaction, followed by exposure to the other reactant. With thetriggering of foam formation, a foam-coated proppant particulate will beproduced.

As an example, a water-blown polyurethane foam can be used to provide acoating around the particles that would lower the overall particledensity. To make such a coated particle, the particle can be initiallycoated with Reactant A, for example a mixture of one or more polyolswith a suitable catalyst (e.g., an amine). This particle can then beexposed to Reactant B containing a diisocyanate. The final foam willform on the particle, for example when it is treated with steam whilebeing shaken; the agitation will prevent the particles fromagglomerating as the foam forms on their surfaces.

In embodiments, fibers, including biodegradable fibers can be added tothe fracture fluid along with SSP. Fibers, including biodegradablefibers, can form a fiber network that help carry the proppant with thefluid. A number of fiber types are familiar to skilled artisans foradding to fracture fluid. As would be understood by skilled artisans,fibers added to the fracture fluid can degrade under downholeconditions, and channels are formed in the proppant pack. The channelsthen have higher permeability and are therefore the flow channelsthrough which hydrocarbons travel from the formation to the wellbore.

The term “fiber” can refer to a synthetic fiber or a natural fiber. Asused herein, the term “synthetic fibers” include fibers or microfibersthat are manufactured in whole or in part. Synthetic fibers includeartificial fibers, where a natural precursor material is modified toform a fiber. For example, cellulose (derived from natural materials)can be formed into an artificial fiber such as Rayon or Lyocell.Cellulose can also be modified to produce cellulose acetate fibers.These artificial fibers are examples of synthetic fibers. Syntheticfibers can be formed from synthetic materials that are inorganic ororganic. Exemplary synthetic fibers can be formed from materials such assubstituted or unsubstituted lactides, glycolides, polylactic acid,polyglycolic acid, or copolymers thereof. Other materials to form fibersinclude polymers of glycolic acid or copolymers formed therewith, as arefamiliar to skilled artisans.

As used herein, the term “natural fiber” refers to a fiber or amicrofiber derived from a natural source without artificialmodification. Natural fibers include vegetable-derived fibers,animal-derived fibers and mineral-derived fibers. Vegetable-derivedfibers can be predominately cellulosic, e.g., cotton, jute, flax, hemp,sisal, ramie, and the like. Vegetable-derived fibers can include fibersderived from seeds or seed cases, such as cotton or kapok.Vegetable-derived fibers can include fibers derived from leaves, such assisal and agave. Vegetable-derived fibers can include fibers derivedfrom the skin or bast surrounding the stem of a plant, such as flax,jute, kenaf, hemp, ramie, rattan, soybean fibers, vine fibers, jute,kenaf, industrial hemp, ramie, rattan, soybean fiber, and banana fibers.Vegetable-derived fibers can include fibers derived from the fruit of aplant, such as coconut fibers. Vegetable-derived fibers can includefibers derived from the stalk of a plant, such as wheat, rice, barley,bamboo, and grass. Vegetable-derived fibers can include wood fibers.Animal-derived fibers typically comprise proteins, e.g., wool, silk,mohair, and the like. Animal-derived fibers can be derived from animalhair, e.g., sheep's wool, goat hair, alpaca hair, horse hair, etc.Animal-derived fibers can be derived from animal body parts, e.g.,catgut, sinew, etc. Animal-derived fibers can be collected from thedried saliva or other excretions of insects or their cocoons, e.g., silkobtained from silk worm cocoons. Animal-derived fibers can be derivedfrom feathers of birds. Mineral-derived natural fibers are obtained fromminerals. Mineral-derived fibers can be derived from asbestos.Mineral-derived fibers can be a glass or ceramic fiber, e.g., glass woolfibers, quartz fibers, aluminum oxide, silicon carbide, boron carbide,and the like.

Fibers may advantageously be selected or formed so that they degrade atspecified pH or temperatures, or to degrade over time, and/or to havechemical compatibilities with specified carrier fluids used in proppanttransport. Useful synthetic fibers can be made, for example, from solidcyclic dimers or solid polymers of organic acids known to hydrolyzeunder specific or tunable conditions of pH, temperature, time, and thelike. Advantageously, fibers can decompose in the locations to whichthey have been transported under predetermined conditioned.Advantageously, the decomposition of the fibers can yield decompositionproducts that are environmentally benign.

EXAMPLES Materials

30/70 mesh frac sand

30/50 mesh frac sand

40/70 mesh frac sand

Polydiallyldimethylammonium chloride (Aldrich, St. Louis, Mo.)

LPEI 500 (Polymer Chemistry Innovations, Tucson, Ariz.)

Ethyl Alcohol, 200 Proof (Aldrich, St. Louis, Mo.)

Hexane (VWR, Radnor, Pa.)

FLOPAM EM533 (SNF)

Polyethyleneglycol diglycidyl ether (Aldrich, St. Louis, Mo.)

Glyoxal, 40 wt % solution (Aldrich, St. Louis, Mo.)

HFC-44 (Polymer Ventures, Charleston, S.C.)

Carboxymethyl Cellulose, sodium salt (Sigma-Aldrich, St. Louis, Mo.)

Ammonium Persulfate (Sigma-Aldrich, St. Louis, Mo.)

Ethoxylated lauryl alcohol surfactant (Ethal LA-12/80%)) (Ethox ChemicalCo, SC)

Glycerol (US Glycerin, Kalamazoo, Mich.)

Potassium Chloride (Morton Salt, Chicago, Ill.)

Fumed Silica (Cabot, Boston, Mass.)

Example 1 Preparation of Inner Polymer Layer

An inner polymer layer of 100 ppm concentration was prepared on a sandsample by adding 200 g 30/70 mesh frac sand to a FlackTek Max 100 longjar. To the sand was added 85 g tap water and 2 g of a 1%polydiallyldimethylammonium chloride (PDAC) solution. The sample wasthen shaken by hand for approximately 5 minutes, vacuum filtered anddried in an oven at 80° C. The sand sample was then removed from theoven and used in subsequent testing.

An identical method was used as described above to formulate a 10 ppminner polymer layer coating with the exception being that only 0.2 g ofa 1% PDAC solution were used.

An identical method was used as described above to formulate an innerpolymer layer at a maximum polymer loading (“Max PDAC”) with theexception that 1 g of a 20 wt % PDAC solution was used. Followingtreatment the sand was washed with excess tap water, vacuum filtered anddried in an oven at 80° C. The sand sample was then removed from theoven and used in subsequent testing.

Example 2 Preparation of Inner Polymer Layer

An inner polymer layer of 100 ppm concentration was prepared on a sandsample by dissolving 0.2 g LPEI 500 in 10 g ethanol to form a 2% LPEI500 solution in ethanol. To 70 g ethanol in a 250 mL round bottom flaskwas added 0.75 g of the 2% LPEI 500 solution. Then 150 g of 30/70 meshfrac sand was added to the round bottom flask. The solvent was removedusing a rotary evaporator with a 65° C. water bath. The sample was thenremoved from the flask and used in subsequent testing.

Example 3 Preparation of Outer Polymer Layer

Outer polymer layers were applied to sand samples by mixing sand withliquid Flopam EM533 polymer under different conditions. In one coatingmethod, polymer product was added neat. In another coating method thepolymer product was extended by diluting with hexane. For hexanedilution 10 g polymer was added to 10 g hexane in a 40 mL glass vial andvortex mixed until homogenous. Polymer was then added to 30/70 mesh fracsand samples of 30 g in FlackTek Max 100 jars. Samples were placed in aFlackTek DAC150 SpeedMixer at 2600 rpm for about 25 seconds. Sampleswere removed from SpeedMixer and allowed to dry in an oven at 80° C.overnight.

Example 4 Performance of Outer Polymer Layer, Settling Times

Sand samples prepared in previous example were assessed for performancein a settling test. Prior to testing, all sand samples were sievedthrough a 25 mesh screen. Settling times were obtained by adding 1 g ofsand sample to 100 mL of tap water in a 100 mL graduated cylinder. Thegraduated cylinder was then inverted about 8 times and then the timerequired for all of the sand to settle at the bottom of the graduatedcylinder was recorded. Three times were recorded for each sample.Settling times are reported in Table 1.

TABLE 1 Outer Treatment Settling Settling Settling Inner Layer AddedTime1 Time2 Time3 Sample Layer Treatment (g) (sec) (sec) (sec) 1 100 ppmFlopam 1 34 35 32 PDAC EM533 2 100 ppm 50:50 2 25 25 26 PDAC FlopamEM533/ hexane 3 100 ppm Flopam 3 35 71 60 PDAC EM533 4 100 ppm 50:50 624 33 32 PDAC Flopam EM533/ hexane 5 Max Flopam 1 19 21 27 PDAC EM533 6Max 50:50 2 17 23 21 PDAC Flopam EM533/ hexane 7 Max Flopam 3 29 31 35PDAC EM533 8 Max 50:50 6 23 24 25 PDAC Flopam EM533/ hexane 9 NoneFlopam 1 22 22 22 EM533 10 None Flopam 3 25 54 64 EM533 11 None None 010 10 10

Example 5 Performance of Outer Polymer Layer, Settled Bed Height

Sand samples prepared in Example 3 with outer polymer layer were alsoassessed by observing the settled bed height in water. In a 20 mL glassvial, 1 g of a sand sample was added to 10 g tap water. The vials wereinverted about 10 times to adequately wet the sand treatments. The vialswere then allowed to sit undisturbed for about 30 minutes. A digitalcaliper was then used to record the height of the sand bed in the vial.Results are reported in Table 2.

TABLE 2 Sample 1 2 3 4 5 6 7 8 9 10 11 Bed 13.5 6.9 22.6 8.9 8.9 5.811.9 n/a 11.9 22.9 0.8 Height (mm)

Example 6 Ionic Crosslink of Outer Polymer Layer

A 40 g 30/70 mesh frac sand sample was treated with an outer polymerlayer by adding 1.3 g Flopam EM533 polymer to the 40 g of sand in aFlackTek Max 100 jar and shaking the jar by hand for 2 minutes. The sandwas then sieved through a 25 mesh screen. To assess polymer retention onsand under shear, tests were conducted by adding 10 g of treated sand to200 g tap water with different levels of PDAC in a 300 mL glass beaker.It is believed that the PDAC will interact ionically to stabilize thepolymer layer on the sand. The slurries were then stirred at 900 rpmwith an overhead mixer using a flat propeller style mixing blade for 5minutes. Mixing was then stopped and samples were allowed to settle for10 minutes. Viscosity of the supernatant was then measured using aBrookfield DV-III+ rheometer with an LV-II spindle at 60 rpm. Bed heightof the settled sand in the beaker was also recorded using a digitalcaliper. Results are reported in Table 3.

TABLE 3 Sample PDAC Conc. (ppm) Visc. (cP) Bed Height (mm) 12 0 25 4.513 60 10 8.6 14 200 2.5 6.3

Example 7 Covalent Crosslink of Outer Polymer Layer—PEGDGE

Four samples of 30/70 mesh frac sand were treated with Flopam EM533 byadding 0.66 g polymer to 20 g sand in a FlackTek Max 100 jar and shakingby hand for 2 minutes. Then various amounts of a fresh 1%polyethyleneglycol diglycidyl ether solution in deionized water wereadded to the treated sand samples. The samples were again shaken by handfor 2 minutes and then placed in an oven at 100° C. for 1 hour. Sampleswere then removed from the oven and sieved through a 25 mesh screen. Bedheights were measured for the four samples by adding 1 g of the sandsample to 10 g of tap water in a 20 mL glass vial, inverting the vialsapproximately 10 times to adequately wet the sand and allowing the vialsto sit undisturbed for about 10 minutes. Bed heights were then measuredwith a digital caliper. Results are listed in Table 4.

TABLE 4 Sample 1% PEGDGE (g) Bed Height (mm) 15 0.1 9.3 16 0.2 8.8 171.0 6.2 18 0 12.7

Example 8 Covalent Crosslink of Outer Polymer Layer—Glyoxal

Four samples of 30/70 mesh frac sand were treated with Flopam EM533 byadding 0.66 g polymer to 20 g sand in a FlackTek Max 100 jar and shakingby hand for 2 minutes. A 1% glyoxal solution in ethanol was formulatedby adding 0.25 g 40 wt % glyoxal to a 20 mL glass vial and diluting to10 g with ethanol. Then varying amounts of the 1% glyoxal solution wereadded to the treated sand samples, and the samples were shaken by handfor 2 minutes and placed in the oven at 100° C. for 30 minutes. The sandsamples were removed from the oven and sieved through a 25 mesh screen.For settled bed height measurements 1 g of sand was added to 10 g tapwater in 20 mL vials, inverted about 10 times and given about 10 minutesto settle. Bed height was measured with a digital caliper. Results arelisted in Table 5.

TABLE 5 Sample 1% glyoxal (g) Bed Height (mm) 19 0.2 3.8 20 0.5 3.5 211.0 2.7 22 2.0 2.7

Example 9 Cationic/Anionic Polymer Treatments

Three samples of 30 g of 30/70 mesh frac sand were treated with PolymerVentures HCF-44 in a FlackTek Max 100 jar. The jar was shaken by handfor 2 minutes. Flopam EM533 was then added to each of the samples. Thejars were again shaken by hand for 2 minutes. The samples were thendried at 80° C. overnight. The sand samples were removed from the ovenand sieved through a 25 mesh screen. For settled bed height measurements1 g of sand was added to 10 g tap water in 20 mL vials, inverted about10 times and given about 10 minutes to settle. Bed height was measuredwith a digital caliper. Results are given in Table 6.

TABLE 6 Sample HCF-44 (g) Flopam EM533 (g) Bed Height (mm) 23 0 0.4510.26 24 0.07 0.38 8.08 25 1.0 0.35 5.08 26 1.5 0.30 3.94

Example 10 Sand Coated with Macromolecular Particles

A 30 g sample of 30/70 mesh frac sand was added to a FlackTek Max 100jar. To the sand, 0.3 g of paraffin wax was added. The sample was placedin a FlackTek DAC 150 SpeedMixer and mixed at 2500 rpm for 2 minutes.After mixing, 1 g of carboxymethyl cellulose was added to the sample.The sample was again placed in the FlackTek DAC 150 SpeedMixer and mixedat 2500 rpm for 1 minute. The sand sample was sieved through a 25 meshscreen. For settled bed height measurements 1 g of sand was added to 10g tap water in a 20 mL vial, inverted about 10 times and given about 10minutes to settle. The sand in this sample clumped together immediatelyand did not disperse in the water, and an accurate measurement of bedheight could not be obtained.

Example 11 Modified Sand Beaker Testing

A 30 g sample of 30/70 mesh frac sand was added to a FlackTek Max 100jar. The sand was treated with Flopam EM533 by adding 0.45 g of thepolymer to the jar and shaking by hand for 2 minutes. The sample wasthen dried at 80° C. overnight. After drying, the sample was removedfrom the oven and sieved over a 25 mesh screen. After sieving, foursamples were prepared by adding 1 g of the treated sand to 10 g of tapwater in a 20 mL vial. The vials were inverted about 10 times and leftto settle for 10 minutes. A 10% ammonium persulfate solution was made byadding 2 g of ammonium persulfate to 18 g of tap water. Varying amountsof the 10% ammonium persulfate solution were then added to the samplevials. The samples were inverted several times to mix, and then placedin an oven at 80° C. for 1 hr. After 1 hour the samples were removed andthe settled bed heights were observed. Table 7 shows the results.

TABLE 7 Sample 10% APS (μL) Sand Suspension 27 0 Suspended 28 180Settled 29 90 Settled 30 18 Settled

Example 12 Emulsion Additives

To determine the effect of emulsion additives on self-suspendingproppant (“SSP”) performance, glycerol and Ethal LA-12/80% were added tothe emulsion polymer EM533 before coating the proppant sand. Threedifferent polymer samples were made as follows:

-   -   SSP Polymer: 10 g of EM533, no additive    -   SSP+glycerol: 9 g EM533 and 1 g of glycerol    -   SSP+glycerol+Ethal: 9 g EM533+0.9 g glycerol+0.1 g Ethal        LA-12/80%

Each of the above samples was vortex mixed for 30 seconds to ensurehomogeneity. To make the modified proppant, 50 g of 40/70 sand wascombined with 1.5 g of one of the polymer samples above and then mixedfor 30 s. The modified proppant samples were evaluated for shearstability in the 1 liter shear test. This test involves addition of 50grams of modified proppant to 1 liter of water in a square plasticbeaker, followed by mixing on a paddle/jar mixer (EC Engineering modelCLM-4) at 200 rpm (corresponding to a shear rate of about 550 s⁻¹) fordifferent amounts of time. The sheared samples are then poured into a1000 mL graduated cylinder and allowed to settle by gravity for 10minutes, then the bed height of the settled proppant sand is recorded.For comparison, an unmodified proppant sand will produce a bed height of10 mm after any amount of mixing. The self-suspending proppant sampleswill produce a higher bed level vs. unmodified proppant due to thehydrogel layer encapsulating the sand grains. Generally, increasing theshear rate or time can cause the bed height of self-suspending proppantto decrease as a result of desorption of the hydrogel layer from thesurface of the modified proppant. For this reason, it is desirable forthe bed height to be as high as possible in this test, especially aftershear. The results below show that the addition of glycerol improves thebed height and the shear stability of the product. The addition ofglycerol and Ethal, while it improves the initial bed height, the longterm shear stability is slightly decreased. These results areillustrated in the graph in FIG. 2.

Example 13 Glycerol and Processability

This experiment sought to determine the effect of glycerol and otheradditives on the performance of self-suspending proppants (denoted asSSP below). 1 kg of dry 40/70 sand was added to the bowl of a KitchenAidstand mixer, model KSM90WH, which was fitted with the paddle attachment.3.09 g of glycerol was mixed with 27.84 g of EM533 emulsion polymer,then the mixture was added to the top of the sand and allowed to soak infor 1 minute. At time 0 the mixer was started at speed 1 (72 rpm primaryrotation). Samples were collected at 1-2 minute intervals and dried for1 hour at 90° C. Then, each sample was subjected to the 1 liter sheartest, where 50 g of SSP was added to 1 L of water and sheared at 200 rpm(an approximate shear rate of 550 s⁻¹) for 20 minutes. Aftertransferring the water/SSP mixture to a 1 liter graduated cylinder andsettling for 10 min, the bed heights were recorded. The experiment wasrepeated with 30.93 g EM533 emulsion polymer alone added to 1 kg ofsand. These results are shown in FIG. 3. As shown in the graph, theglycerol additive increased bed heights significantly.

The difference in performance was even more marked when the experimentwas repeated at higher mixing speeds. Here the mixer was set to speed 4(150 rpm primary rotation). At low mixing times, the samples wereinsufficiently mixed, leading to incomplete coating of the sand andready desorption of the polymer from the surface of the SSP during theshear test. As mixing time of the coating process increased so didperformance, until an ideal coating was reached, giving maximum bedheight for that sample. After that, increasingly worse (lower) bedheights were seen at higher mixing times, possibly as a result ofabrasion of the coating during extended mixing. At higher mixing speeds,this process happened even faster, such that the processing window forthe emulsion polymer alone was less than 1 minute. With the addition ofglycerol and the use of a lower mixing speed, this processing window waswidened to nearly 15 minutes. In comparison to the tests with emulsionpolymer alone, glycerol caused the processing window to widen,indicating that SSP preparation with the glycerol is more robust. At thesame time, glycerol allowed the polymer emulsion to invert more fully,leading to better coatings and increased bed heights. Testing withcombinations of glycerol and the emulsion polymer EM533 at a highermixing speed yielded the results shown in FIG. 4.

Example 14 Modified Proppant with an Anticaking Agent

Modified proppant samples were made with and without anticaking agentfor a comparison. For Sample A, 50 g of 40/70 sand was added to aFlackTek jar. 1.5 g of EM533 emulsion polymer was added to the sand andthe sample was mixed for 30 seconds. After mixing, 0.25 g of calciumsilicate was added to the sample and the sample was mixed again for 30seconds. The sample was then dried for 1 hour at 85° C. After drying,the sample was poured over a 25 mesh screen and shaken lightly for 30seconds. The amount of sand that passed through the sieve was thenmeasured. For Sample B, 50 g of 40/70 sand was added to a FlackTek jar.1.5 g of EM533 emulsion polymer was added to the sand and the sample wasmixed for 30 seconds. The sample was then dried for 1 hour at 85° C.After drying, the sample was poured over a 25 mesh screen and shakenlightly for 30 seconds. The amount of sand that passed through the sievewas then measured. Table 8 shows the results.

TABLE 8 Total Mass Mass passing % Passing Sample Sample, g Sieve, gSieve Sample A 50.5 50.16 99.3% Sample B 50.5 15.71 31.1%

The results of sieve testing show that the incorporation of ananticaking agent was effective at improving the handling properties ofthe modified proppants.

Samples A and B were separately added to 1 L of water and then shearedin the EC Engineering Mixer for 20 minutes at 200 rpm. After shearing,the samples were transferred to a 1 L graduated cylinder and left tosettle for 10 minutes. After settling, the bed heights were measured,showing no significant loss in shear stability as a result ofincorporating an anticaking agent. Table 9 shows these results.

TABLE 9 Bed Height, Sample mm Sample A 56.21 Sample B 59.67

Example 15 Coating of Proppant with Hydrogel Layer

A coating composition was made by combining 10 g glycerol and 90 gFlopam EM533 in a glass vial and mixing for 30 seconds with a vortexmixer. Next, 400 g of 40/70 mesh proppant sand was added to a KitchenAidmixer bowl. 16 g of the coating composition was added to the KitchenAidmixer bowl. The mixer was then turned on to the lowest setting and leftto mix for 7 minutes. After mixing, the sand was split into 50 g samplesand placed in a forced air oven at 80° C. for 1 hr. After drying, themodified proppant was screened through a 25 mesh sieve.

Example 16 Coating of Proppant with Hydrogel Layer

400 g of 40/70 proppant sand was added to a KitchenAid mixer bowl. 16 gof SNF Flopam EM533 was added to the KitchenAid mixer bowl. The mixerwas then turned on to the lowest setting and left to mix for 7 minutes.After mixing, the sand was split into 50 g samples and placed in aforced air oven at 80° C. for 1 hr. After drying, the modified proppantwas screened through a 25 mesh sieve.

Example 17 Shear Stability Testing

Coated sand samples made in Examples 15 and 16 were tested for shearstability. 1 L of tap water was added to a square 1 L beaker. The beakerwas then placed in an EC Engineering CLM4 paddle mixer. The mixer wasset to mix at 300 rpm. Once mixing commenced, 50 g of the coated sandsample was added to the beaker. After 30 seconds of mixing at 300 rpm,the mixing was reduced to 200 rpm and continued for 20 minutes. At theend of this mixing, the mixture was poured into a 1 L graduated cylinderand allowed to settle. After 10 minutes, the settled bed height wasrecorded, as shown in Table 10. Higher bed heights indicate betterproppant performance.

TABLE 10 Sand Sample Bed Height after shear, mm Untreated 40/70 Sand13.24 Example 2 70.4 Example 3 57.64

Example 18 Brine Tolerance

Two 20 mL vials were filled with 10 mL of tap water. Separately, anothertwo 20 mL vials were filled with 10 mL of a 1% KCl solution. 1 g of sandprepared in Example 15 was added to a vial containing tap water and 1 gwas added to a vial containing 1% KCl. Also, 1 g of sand prepared inExample 6 was added to a vial containing tap water and 1 g was added toa vial containing 1% KCl. All four vials were inverted ˜7 times and thenleft to settle for 10 minutes. After settling, the bed heights weremeasured. The results are shown in Table 11.

TABLE 11 Sand Tap Water Bed Height, 1% KCl Bed Sample mm Height, mmExample 2 10.39 5.02 Example 6 17.15 9.23

Example 19 Abrasion Testing

Three 250 mL beakers were filled with 50 mL of tap water. One aluminumdisk with a mass of about 5.5-6 g was placed in each of the beakers. One2 inch stir bar was placed in each of the beakers as well. All threebeakers were placed their own magnetic stir plates and the plates wereset to speed setting 5. Six grams of 40/70 sand was added to one of thebeakers. Six grams of sand prepared in Example 15 was placed a secondbeaker. The third beaker had no sand added at all. Each of the beakerswas left to stir for 2 hours. After stirring, the aluminum disk wasremoved, washed and then dried. The mass was then measured again. Theresults, shown in Table 12, indicate that the sand prepared in Example15 results in less abrasion to metal surfaces upon contact, comparedwith unmodified sand.

TABLE 12 Initial Total Mass, g Mass After 2 hrs, g Loss, g % Loss NoSand 5.62 5.612 0.008 0.14% 40/70 Sand, 6.044 6.027 0.017 0.28% uncoatedExample 15 5.673 5.671 0.002 0.04% Sand

Example 20 Effect of Glycerol on Mixing

1 kg of dry 40/70 sand was added to the bowl of a KitchenAid standmixer, model KSM90WH, which was fitted with the paddle attachment. 3.09g of glycerol was mixed with 27.84 g of emulsion polymer then themixture was added to the top of the sand and allowed to soak in for 1minute. At time 0 the mixer was started at speed 4 (150 rpm primaryrotation). Samples were collected at 1-2 minute intervals and dried for1 hour at 90° C. Then, each sample was subjected to a shear test, where50 g of SSP was added to 1 L of water and sheared at 550 s⁻¹ for 20minutes. After settling for 10 min, the bed heights were recorded. Theresults of these shear tests are shown in FIG. 5. The graph demonstratesthat both undermixing and overmixing can affect the behavior of thecoated proppants, leading to dissociation of the polymer from the sandduring the shear test. In this example, an optimal amount of mixing wasbetween about 5 and 20 minutes. The effect of mixing duration uponperformance suggests that the coating is fragile while wet, and it ismore durable once it is dry. In comparison to the coating tests withemulsion polymer alone, coatings with glycerol-blended emulsionsappeared to cause the processing window (i.e., the acceptable amount ofmixing time) to widen. Additionally, glycerol-blended emulsion coatingsappeared to invert more fully, leading to better coating properties suchas increased bed heights.

Example 21 Production of Self-Suspending Proppant Using a Pug Mill

A 3 cubic foot pug mill type mixer was used to make a batch ofself-suspending proppant. About 50 lbs of 40/70 mesh sand was added tothe pug mill. In a 1 L beaker, about 756 g of SNF Flopam EM533 was addedand 84 g of glycerol was mixed into the polymer. The entire mixture wasthen poured evenly on top of the sand in the pug mill. The pug mill wasturned on and mixed at about 70 rpm. Samples were taken after 30, 60,120, 180, 240, 300, 420, and 600 seconds of mixing. The samples weredried for one hour. After drying, the 50 g of each sample was added to 1L of water and mixed in an EC Engineering CLM4 for 20 min at 200 rpm.After mixing, the sample was poured into a 1 L graduated cylinder andallowed to settle for 10 minutes. After settling, the bed height wasmeasured. The results are shown in Table 13.

TABLE 13 Pug Mill Mixing Time (sec) Bed Height, mm 30 29.34 60 23.49 12048.9 180 57.58 240 55.71 300 44.88 420 57.21 600 57.25

Example 22 Wet Aging

A 400 g sample of self-suspending proppant (SSP) was manufactured in thesame manner as Example 15. The 400 g of SSP was split into 50 g samplesand left in closed containers and left at room temperature. After dryingfor various amounts of time, the samples were tested in the same manneras Example 21. The results are shown in Table 14.

TABLE 14 Aging Time, hr Final Bed Height, mm 0 10.1 2 26.63 4 60.16

Example 23 SSP Plus Uncoated Proppant

10 mL of tap water was added to a 20 mL vial. Proppant sand, both SSPprepared in accordance with Example 15 and unmodified 40/70 was thenadded to the vial. The vial was inverted several times and then left tosettle for 10 minutes. After settling, the bed height was measured. Theresults are shown in Table 15.

TABLE 15 SSP, grams 40/70 Sand, grams Settled Bed Height, mm 0.5 0.55.46 0.75 0.25 5.71 0.9 0.1 8.23

Example 24 Anti-Caking Agents Added to SSP

A 400 g batch of SSP was produced in the same manner as described inExample 15. The sample was split into about 50 g subsamples and then0.25 g of fumed silica with an aggregate size of 80 nm was mixed intoeach sample. Samples were then covered and aged at room temperature. Thesamples were tested in the same manner as described in Example 21. Theresults are shown in Table 16.

TABLE 16 Hours Aging Settled Bed Height, mm 18 57.3 24 41.28 42 44.29 4844.76 72 45.48

Example 25 Respirable Dust

200 g samples of uncoated and hydrogel-coated sand (40/70 mesh) preparedaccording to Example 15 were sieved with a 140 mesh screen, and the fineparticulates that pass through the 140 mesh sieve were collected andweighed. The coated sample of sand demonstrated an 86% reduction on theamount of fine particulates relative to the uncoated sample of sand. Theresults are shown in Table 17.

TABLE 17 Weight of the Weight of Dust Percentage sample the dust oftotal Uncoated sample 200.011 g 0.0779 g 0.03895% Coated sample 200.005g 0.0108 g 0.00540%

Example 26 Anti-Caking Agents with Different Particulate Size

50 g of 40/70 mesh sand was mixed with 2 g of SNF Flopam EM533 using thespeed mixer for 30 seconds at 800 rpm. Then 0.625 g of an anti-cakingagent was added and the material was again mixed in the speed mixer for30 seconds. The samples were allowed to sit for 3 hours, then tested ina 20 min shear test, allowed to settle for 10 min and the bed heightmeasured. Results are shown in Table 18. The anti-caking agents improvedthe bed height after shear testing with a wide range of particle sizes.

TABLE 18 Anti-caking agent Particulate Size Bed Height (mm) Talc(magnesium 12 microns 16.76 silicate) Calcium Silicate 1-3 microns 39.78Fumed Silica (EH-5) 80 nanometers 73.87

Example 27 Chemical Composition of Anti-Caking Agents

A wide variety of anti-caking agents were tested, as listed in Table 19.For each agent tested, 700 g of 40/70 sand was mixed in the KitchenAidmixer at speed 1 (144 rpm) with 21.65 g of a 10% glycerol/90% EM533mixture. 50 g samples were separated out and mixed with the appropriateamount of anti-caking agent in the speed mixer. Three samples, whichwere mixed with 1% calcium silicate, 1.5% diatomaceous earth, and 1.5%Kaolin respectively, were tested in the shear test immediately, whilethe other 7 samples were dried for 1 hour in an 80° C. oven along with acontrol sample with no anti-caking agent. All samples were tested in thesame manner as Example 17. Table 19-A shows bed heights after sheartesting wet (non-dried) samples with an anti-caking agent applied. Table19-B shows bed height after shear testing of dried (1 hr at 80° C.)samples with anti-caking agent applied.

TABLE 19-A Anti-Caking Agent Amount Bed Height (mm) Calcium Silicate 0.5 g (1%) 30.26 Diatomaceous earth (DE) 0.75 g (1.5%) 12.95 Kaolinclay 0.75 g (1.5%) 18.46

TABLE 19-B Anti-Caking Agent Amount Bed Height (mm) NONE — 85.9 SodiumBicarbonate 0.5 g (1%) 56.97 Cornstarch 0.5 g (1%) 32.29 Baby Powder(talc) 0.5 g (1%) 84.83 Dry-Floc AF (hydrophobic 0.5 g (1%) 32.24starch) Tapioca Maltodextrin 0.5 g (1%) 27.08 Microcrystalline cellulose0.5 g (1%) 40.12 Baking Powder 0.5 g (1%) 39.88

Example 28 Anti-Caking Agents: Amounts Needed for Drying

Seven 50 g samples of 40/70 sand were added to small plastic jars,followed by 2 g each of 10% glycerol/90% emulsion polymer mixture foreach. After speed mixing for 30 seconds, 0.25 g, 0.375 g, 0.5 g, 0.675g, 0.75 g, 1 g and 2.5 g of calcium silicate powder were added to theseven samples and the sand was again mixed for 30 seconds. The sampleswere shear tested without a further drying step, and the settled bedheight was recorded in mm. The results are shown in FIG. 6. A similarexperiment was carried out using silica as an anticaking agent. Thesetests showed that a sand coated with a hydrogel can be treated with ananticaking agent, yielding a product that does not require a separatedrying step to produce an acceptable bed height after shear testing.

Example 29 Silica Anti-Caking Agents

50 g of 40/70 sand was added to a small jar, followed by 2 g of 10%glycerol/90% EM533. The jar was speed mixed at 800 rpm for 30 seconds,then the appropriate amount of fumed silica was added, as described inTable 20, and it was mixed for another 30 sec. The samples underwent a20 min shear test and the bed heights were recorded. No oven drying wasused. Results are shown in Table 20.

TABLE 20 Compound name Chemical character Amount added Bed Height EH-5Amorphous fumed 1% 136.25 mm silica M-5 Untreated fumed silica 1% 123.52mm TS-720 Treated fumed silica, 1%  26.21 mm siloxanes and siliconesPG001 30% anionic colloidal 1% solids  15.30 mm silica, 25.9% solids

A batch of coated sand was mixed in the KitchenAid mixer and separatedinto several 50 g samples. Then 1 wt % of various sizes of fumed silicawere added to each of 3 samples, mixed, and shear tested. These testresults are shown in Table 21.

TABLE 21 Powder Approx. Size Amount added Bed height Aldrich Fumed  7 nm1% 48.86 mm Silica Aldrich Silica 10 nm 1% 35.48 mm Nanopowder CabotEH-5 80 nm aggregates 1% 59.10 mm

Example 30 Preheating Sand

500 g of 30/50 sand was placed in a 90° C. oven for 1 h with occasionalstirring, until the temperature of the sand equilibrated. The sand wasthen mixed in a commercial planetary mixer until it reached the desiredpre-heated temperature (45° C., 60° C. or 80° C.), at which point 20.8 gof the SNF Flopam EM533 was added and the sample mixed for 7 min. Thebatch was then divided and dried in the oven for a range of times at 80°C. For the non-preheated samples, 500 g of 30/50 sand was placed in themixer bowl with 20.8 g of polymer emulsion, mixed for 7 minutes, andthen dried for varying amounts of time. All samples were shear testedusing the standard procedure: 50 g of sand added to 1000 g of tap water,stirred at a shear rate of 550 s⁻¹ for 20 minutes, then settled for 10min in a 1 L graduated cylinder. The results are shown in FIG. 7. Theseresults suggest that preheating the sand to 45° C. is acceptable but60-80° C. results in lower bed height in shear tests.

Example 31 Forced Air Drying

50 g of 40/70 sand was mixed with of 4% emulsion polymer (2 g) preparedaccording to Example 14 using the speed mixer for 30 seconds. The samplewas transferred to a container fitted with a hot air gun set at 90, 95or 100° C. The sample was left under the heat gun for 30 min total, with5 g samples taken out at the 5, 10, 15 and 30 min marks. These sampleswere then tested using the Small shear test, performed as follows: 100mL of tap water was set stirring in a 300 mL beaker using a 2 inch stirbar spinning at 500 rpm; 5 g of the sand sample was added to the beakerand sheared for 3 minutes; the whole solution was transferred to a 100mL graduated cylinder, inverted once, settled for 5 minutes, and the bedheight measured. The results of these tests are shown in FIG. 8. Asshown in the graph, higher temperatures of the incoming forced aircaused more complete drying and better bed height. To test thesusceptibility of SSP to shear while drying with forced air, a sevenprong rake was pulled back and forth through the sample to simulatelight shear while drying. Two 50 g batches of SSP were prepared anddried under 110° C. air for 30 min. The first was completely static,while the second was constantly raked during the 30 min dry time. Bothsamples were tested using the large shear test for 20 min with asettling time of 10 min. The sample with static drying gave a settledbed volume of 100.63 mm; while the sample dried with light shear gave asettled bed volume of 109.49 mm.

Example 32 Mixing with Vertical Screw

A small-scale vertical screw blender was constructed. Sand and SNFFlopam EM533 were added to the container, and then mixed with the screwturning at about 120 rpm. The sample was then split into two 50 g parts,one of which was oven dried at 80° C., the other mixed with 0.5 g (1 wt%) fumed silica. Both were then subjected to a shear test as describedin Example 17. The results of bed height measurement were as follows:Oven Dried, 1 h gave a bed height of 101.34 mm; Undried, with 1% of 7 nmfumed silica added, gave a bed height of 91.47 mm. Both oven drying andthe addition of anti-caking agent to dry the product produced high bedheights.

Example 33 Microwave Drying

50 g of 40/70 sand was added to a small plastic jar, and then mixed with2 g of a blend containing (10% glycerol/90% emulsion polymer) in thespeed mixer for 30 seconds at 800 rpm. The sample was placed in a 700 Wmicrowave oven and heated on high for 45 seconds. The sample was sievedand cooled, then sheared at 200 rpm for 20 min in an EC Engineering CLM4mixer. After mixing the sample was transferred to a 1 L beaker and leftto settle for 10 minutes. After settling, the bed height was measured inmillimeters, giving a bed height of 52.43 mm. Microwave heating givesacceptable bed heights with relatively short drying times.

Example 34 Mixing and Heating with Anti-Caking Agents

500 g 40/70 sand was mixed in a KitchenAid mixer with 20 g of (20%Glycerol/80% emulsion) for 8 min. Next was added 0.44% of Cabot EH-5fumed silica and mixed for 2 minutes, and then the sample was heatedwith the heat gun. 50 g samples were collected at 13, 18, 24 and 26minutes of mixing time. These were shear tested for 20 min and the bedheights recorded. The results are shown in FIG. 9. A combination ofglycerol and silica made the processing window longer.

Example 35 Microwave Drying

400 g of 30/50 sand was combined with 16 g (4% wt) of emulsion polymerprepared according to Example 14 and mixed in a KitchenAid stand mixerfor 7 minutes. One 50 g sample was dried using the oven (80° C.), and 7other samples were placed in a 700 W microwave oven for 5, 10, 20, 30,45, 60 and 120 seconds respectively. Shear tests (20 minutes long) asdescribed in Example 12 and loss on ignition (LOI) tests were run on thesamples. An LOI test consisted of adding 10 g of sand to a taredcrucible, which was placed in a 960° C. oven for 1 hour. After heatingfor an hour, the crucible was cooled in a dessicator for 1 hour thenweighed. Drying time, bed height and LOI are shown on Table 22. Thedifference between the initial and final weights was expressed as apercentage of the total initial sand weight, as shown in FIG. 10.

TABLE 22 Drying Method Drying time Bed height (mm) LOI (%) Oven  1 h41.36 1.8 Microwave  5 sec 15.54 3.33 Microwave  10 sec 16.14 Microwave 20 sec 24.68 Microwave  30 sec 39.99 2.929 Microwave  45 sec 53.31Microwave  60 sec 49.84 Microwave 120 sec 57.81 2.279

These tests suggest that the microwave drying technique removespredominantly the water, rather than the oil, from the coated samples.

Example 36 Vacuum Drying

250 g of 30/50 sand were combined with 10 g emulsion polymer asdescribed in Example 14. The sand mixture was stirred in a KitchenAidstand mixer on lowest speed for 7 minutes, then separated into 50 gsamples and dried in a vacuum oven under 24 inches Hg vacuum at 25° C.,50° C. and 85° C. for 1 hour, 1 hour, and 30 minutes respectively. Thesamples were cooled to room temperature, sieved, and shear tested (asdescribed in Example 12) for 20 minutes. The results are shown in Table23.

TABLE 23 Sample # Temperature (° C.) Time Bed Height (mm) 1 25  1 hour16.79 2 50  1 hour 17.34 3 85 30 min 18.04

During these tests, none of the samples dried completely, althoughfurther testing may show that higher temperatures can effect morecomplete drying.

EQUIVALENTS

While specific embodiments of the subject invention have been disclosedherein, the above specification is illustrative and not restrictive.While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims. Many variations of the inventionwill become apparent to those of skilled art upon review of thisspecification. Unless otherwise indicated, all numbers expressingreaction conditions, quantities of ingredients, and so forth, as used inthis specification and the claims are to be understood as being modifiedin all instances by the term “about.” Accordingly, unless indicated tothe contrary, the numerical parameters set forth herein areapproximations that can vary depending upon the desired propertiessought to be obtained by the present invention.

We claim:
 1. An aqueous fracturing fluid which is made by combining anaqueous carrier liquid and a mass of discrete proppant particles, thediscrete proppant particles comprising a modified proppant comprising aproppant particle substrate and a hydrogel coating on the proppantparticle substrate, wherein the hydrogel coating comprises an anionicpolyacrylamide which has been crosslinked by means of a subsequentlyapplied cross-linker selected from the group consisting of an epoxide,an anhydride, an aldehyde, a diisocyanate and a carbodiamide in anamount sufficient to prevent premature hydration of the modifiedproppant due to humidity prior to the combining of the modified proppantwith the aqueous carrier liquid forming the aqueous fracturing fluid,wherein 1 gram of the modified proppant when mixed with water in a 20 mlglass vial and allowed to sit undisturbed for 30 minutes exhibits anexpanded settled bed height which is about 3 (2.7/0.8) to about 28(22.6/0.8) times greater than the settled bed height of an otherwiseidentical proppant not modified with a hydrogel coating, and wherein themodified proppant substantially retains its expanded settled bed heightafter having been subjected to shear at a shear rate of about 550 s⁻¹for 20 minutes.
 2. The aqueous fracturing fluid of claim 1, wherein thecross-linking agent is a diisocyanate.
 3. The aqueous fracturing fluidof claim 1, wherein the aqueous fracturing fluid is free of aviscosity-increasing polymer.
 4. The aqueous fracturing fluid of claim1, wherein the aqueous fracturing fluid contains a viscosity-increasingpolymer to reduce the rate at which the modified proppant settles in thefracturing fluid, and further wherein the amount of thisviscosity-increasing polymer is less than would otherwise be necessaryto achieve the same rate of proppant settling in an otherwise identicalcontrol aqueous fracturing fluid made with an uncoated proppantparticle.
 5. A method of fracturing a subterranean formation comprisingcharging the aqueous fracturing fluid of claim 1 into the formation. 6.The method of claim 5, further comprising contacting the modifiedproppant with a chemical breaker in an amount sufficient to degrade itshydrogel coating.
 7. The aqueous fracturing fluid of claim 1, whereinthe hydrogel-forming polymer coating is applied by combining a proppantparticle substrate with a liquid polymer formulation comprising thehydrogel-forming polymer and a carrier liquid including water followedby drying to remove the water.
 8. The aqueous fracturing fluid of claim7, wherein the carrier liquid which is used to apply thehydrogel-forming polymer to the proppant particle substrate furthercomprises an organic liquid.
 9. The aqueous fracturing fluid of claim 1,wherein the hydrogel coating on the proppant particle substrate includesan alcohol.
 10. The aqueous fracturing fluid of claim 9, wherein thealcohol is at least one of ethanol, propanol, ethylene glycol, propyleneglycol and glycerol.
 11. The aqueous fracturing fluid of claim 10,wherein the alcohol is at least one of ethylene glycol, propylene glycoland glycerol.
 12. The aqueous fracturing fluid of claim 11, wherein thealcohol is glycerol.
 13. The aqueous fracturing fluid of claim 1,wherein the proppant particle substrate is at least one of graded sand,resin coated sand, bauxite, ceramic materials and glass materials.